The NASA Extreme Environment Mission Operations (NEEMO) 21 mission began on July 21, 2016, as an international crew of aquanauts splashed down to the undersea Aquarius Reef Base, located 62 feet below the surface of the Atlantic Ocean in the Florida Keys National Marine Sanctuary. The NEEMO 21 crew will perform research both inside and outside the habitat during a 16-day simulated space mission. During simulated spacewalks carried out underwater, they will evaluate tools and mission operation techniques that could be used in future space missions. Inside the habitat, the crew's objectives include testing a DNA sequencer, a medical telemetry device, and HoloLens operational performance for human spaceflight cargo transfer.